This invention relates to molded articles comprising fiber reinforced styrene polymers which are resistant to heat distortion, and to a method of manufacturing such articles. More particularly the invention is directed to molded articles comprising styrene polymers reinforced with a high concentration of glass fiber, and to a method of manufacturing such articles by a transfer molding process.
Molded articles comprising styrene polymers reinforced with moderate amounts of glass fiber to provide a modest increase in the heat distortion temperature have been prepared by injection molding methods. Unsaturated polyesters and solutions of unsaturated polyesters in styrene monomer have been applied to glass reinforcement to provide molded articles which are hard, and have flexural strength and heat distortion resistance. However such unsaturated polyester systems are high in viscosity and must be applied at high pressure and temperature to provide void free moldings. Moreover the molded articles obtained from them tend to be brittle and to require a post-curing step to complete the cure of the unsaturated groups.
I have now found that molded articles comprising styrene homopolymer or styrene acrylonitrile copolymer reinforced with at least about 55 weight percent of a reinforcing fiber, prepared by a monomer transfer molding process, possess toughness and resistance to heat distortion and do not require a post-curing step.
The molded articles comprise a fiber-reinforced styrene polymer wherein the weight ratio of the fiber reinforcement to the styrene polymer is in the range of 55:45 to 75:25, wherein the fiber reinforcement is a mat or weave of long glass fibers. The molded articles generally have a heat distortion temperature determined under a load of 264 psi (1819 kPa) above about 200.degree. C.
The styrene polymer is a homopolymer or a copolymer of styrene and acrylonitrile comprising up to 50 weight percent acrylonitrile. The styrene polymer is advantageously prepared by fitting a weave or mat of long fibers of glass, into the cavity of a transfer mold, closing and clamping the mold, filling the free space in the mold with the monomer or monomer mixture containing a free radical initiator, heating the mold to a temperature which allows polymerization of the monomers initiated by the free radical initiator to proceed to about a 98 percent conversion and opening the mold and removing the molded article. The time required for polymerization is generally less than about 30 minutes.
The fiber-reinforcement of the molded article is a weave or mat of fibers of at least about 25 mm in length and of diameter in the range of about 5 to about 25 microns. The glass fiber mat is formed from chopped strands of fiber which have been treated with a coupling agent such as vinyltriethoxysilane, vinyltrimethoxysilane, methacrylotrimethoxysilane, or methacrylochromium chloride. The mat is sized with from about 1 to about 5 weight percent of a binder comprising an unsaturated polyester of molecular weight in the range of about 1000 to about 10000, prepared by condensation of a mixture of saturated and unsaturated dicarboxylic acids and glycols of the type conventionally used in the manufacture of such unsaturated polyesters, the unsaturated acids being maleic and fumaric acids present in a concentration of about 10 to about 100 mole percent of the total dicarboxylic acid. Suitable glycols include ethylene glycol, propylene glycol, diethylene glycol, polyethylene glycols, polypropylene glycols and poly(tetramethylene glycols) of molecular up to 400, 1,6-hexanediol, and 1,4-dimethylol cyclohexane. Suitable dicarboxylic acids include phthalic acid, isophthalic acid, terephthalic acid, succinic acid, adipic acid and azelaic acid. Preferably the unsaturated polyester has a melting point in the range of about 50.degree. to about 150.degree. C.
The mold used in the transfer molding process need not be designed for high pressure as is required for injection molding. The molds are advantageously made of low priced metals or fiber reinforced thermoset resins backed by metal supports of designs suitable for withstanding 350-2100 kPa pressure. The mixture of monomers is introduced into the mold by pumping or other suitable means.
To provide a polymerization time less than about 30 minutes, the particular initiator or combination of initiators, and the amount thereof will vary depending on the monomer(s) used. For styrene, for example, a commonly used initiator is tert.-butyl perbenzoate advantageously in the amount of about 0.8 to about 1.0 mole percent. For mixtures of styrene and acrylonitrile it can be advantageous to use a combination of two or more peroxides whose decomposition temperatures differ by 20.degree. C. or more, for example, 0.4 mole percent each of lauroyl and benzoyl peroxides and tert-butyl perbenzoate. The choice of initiator(s) should be made on the basis of reaction temperature used, the volatility of the monomers and the decomposition temperature of the initiator.
The concentration of free radical initiator(s) is advantageously in the range from about 0.4 to about 1.5 mole % of the monomer composition. The preferred temperatures of reaction are about 90.degree.-110.degree. C. for styrene acrylonitrile monomer mixtures. This range is preferred because of the high vapor pressure of acrylonitrile. When styrene is used alone, temperatures of about 90.degree.-150.degree. C. are preferred.
Other elements which may be present in the compositions and are conveniently added in the monomer mixture are promoters such as conventional metal driers for example cobalt naphthenate to increase the rate of polymerization. Fillers such as clay, talc, glass spheres, mica, etc. may be added as partial replacement for the fiber reinforcement provided that the total fiber and filler content is maintained in the weight range of about 25 to about 45 percent and and provided further that no more than 20 weight percent of fiber reinforcement is replaced with filler. Stabilizers, release agents, such as wax and pigments, etc. may also be added. The materials and process are useful for making highly glass loaded, economical composites with high heat resistance and excellent stiffness and strength.